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Full text of "Handbook Of Chemical Engineering - I"

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244                               CHEMICAL ENGINEERING
screen.    Screening en masse the first principal action is an interstitial one which brings the small grains to the screen first followed by successively larger ones.
To understand this action reference should be made to Figs. 16 and 17.    Figure 16 shows diagrammatically the amount of interstitial opening or amount the grains part from one another under the shaking action, A--B would represent the amount of opening at the top of the mass of material on the screen,' while at i        I                 0 where the mass is in contact with the screen the amount the
grains part from one another is zero or very nearly zero.    A particle   such   as G would  pass  down the  interstitial   opening thus created to the position C-D, and evidently fine particles would proceed down still further, until a particle of no sensible diameter would arrive at the surface of the screen.    Under the action of a shaking screen the grains tend to assume the arrangement shown in Fig.  17.    To some extent this arrangement is altered by the position of the grains as they reach the screen but Q      it tends to assume the arrangement very quickly once it is under _      the influence of the screening action.    The shaking action actually displaces the larger grains and brings them to the top of the bed on 0      the screen.    A useful wrinkle -in placing nut coal on a new or 0      nearly dead fire, which needs coal free from fines, depends upon Q              0       giving the coal scuttle a few vigorous shakes which will settle
FIG 16. FIG. 17. down the fines and bring the larger pieces to the top. Interstitial action. So long as there is good interstitial effect, screening with a deep bed will yield a result sufficiently good for most commercial screening problems with non-coherent grains not too finely divided. Where extra good work is not desired, capacity will be the most important factor and as is self evident, other factors being equal the deeper the bed of material on the screen the greater the capacity. The criterion of the heaviest load a flat shaking screen could carry and give fair commercial results in precision of screening would be that at the end of the screen the layer of grains discharging would not be more than one deep. This implies the maximum amount of interference of the grains with one another which should be permitted even in ordinary commercial work. It will be understood that at entry point of the screen the material is piled up much higher than at the discharge point, there being progressive reduction in the depth of the bed as the material fed proceeds over the screen from the elimination of the undersize.
For the most precise work on flat shaking screens the bed should go on the screen in as thin a sheet as possible. The depth of bed should not be greater than the width of aperture when the material first reaches the screen. This will provide sufficient freedom of movement to the undersize grains seeking an outlet to satisfy the highest standards of screen work provided the screen is of sufficient length. In the portion of this chapter on the capacity of shaking screens to be taken up later the proper entry depth of bed for shaking screens and the proper length m are translated into capacity per square foot of screen surface.
Proper Motion in Shaking Screens.—Any sort of mechanism which will give the screen a differential back and forth motion, the screen being kept in a more or less horizontal position while in motion from the actuating mechanism, will give a good screening effect. Horizontal flat shaking screens are mounted on rockers or slides or are suspended by rods. The introduction of mechanical elements to give the screen an upward stroke adds complexity and vibration, Jigging and turning over effect and aid in relieving the screen of blinding, but do not compensate for these disadvantages. The numerous patents of George